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PARIS (Reuters) - Paris enforced the most drastic traffic curbs in 20 years on Monday, fining almost 4,000 drivers who tried the dodge them, but stopping at a single day of restrictions as cooler weather brought relief from pollution.

Transport chiefs made public transport free in the French capital, while drivers with even-numbered license plates were told to leave their cars at home.

Last week's unseasonably hot and windless weather across western Europe left cooler air containing tiny particles from car emissions and other sources trapped under a warmer layer.

French policies to promote the use of diesel is seen as one factor why Paris is hit worse than other cities.

Some 700 police officers posted around the city fined 3,859 people for failing to respect the ban by mid-morning. Congestion was 60 percent lower than usual thanks to traffic volume that was down by a quarter, police said.

Car-sharing web site e-loue.com reported hundreds of requests to hire odd-numbered cars on Monday. A long list of exemptions from the ban included delivery drivers, taxis, and cars carrying at least three people, but the government hailed the move as a success.

"This is a public health problem ... we thank everyone who fell into line," Transport Minister Frederic Cuvillier said.

Drivers who defied the curbs were fined 22 euros ($31) on the spot. The anti-smog action is being implemented days before voters elect city hall mayors across the country in a two-round ballot on March 23 and 30. President Francois Hollande's ruling Socialists are seen retain control of the French capital.

Policemen at the checkpoints said commuters were taking the measures well - even though 27 people had their cars impounded because of their reaction to the fine.

"Most of the road users understand well that this operation has a direct impact on curbing pollution," said police officer Jean-Pierre Meutelet at Porte Maillot on the northwestern edge of Paris.

The government decided against imposing a second day of curbs that would have banned odd-numbered plates from the streets in their turn, saying a change in weather and the traffic curbs had helped ease pollution.

Paris is more prone to smog than other European capitals, World Health Organisation (WHO) figures from 2008 show.

A French tax regime that favors diesel over gasoline is regularly cited as a primary cause. New registrations of French cars in 2012 were 67 percent diesel-powered compared with western European average of 53.3 percent - although the proportion was higher in Ireland at 72 percent and similar in Spain and Belgium at 66.3 and 64.8 percent respectively.

Denis Baupin, a Green Party MP for Paris who has long campaigned for a change in the tax system, said it was time diesel support was scrapped as emission standards tighten and other fuels improve their efficiency.

"The increase in environmental constraints has sent the competitiveness of this technology into sharp decline," he told Reuters.

While nowhere near levels seen in some Asian cities, figures from the European Environment Agency (EEA) web site showed last week 147 micrograms of particulate matter (PM) per cubic meter of air in Paris - compared with 114 in Brussels, 104 in Amsterdam, 81 in Berlin and 79.7 in London.

The last restricted Paris driving scheme was introduced in 1997. It also lasted one day.